Shulamith Firestone, whose 1970 book "The Dialectic of Sex" became a feminist classic with its calls for a drastic rethinking of women's roles in the bearing and raising of children, was found dead Tuesday in her New York City apartment. She was 67.

A recluse who struggled with mental illness in later years, the author apparently died of natural causes, said her sister, Miriam Tirzah Firestone.

Only 25 when "The Dialectic of Sex" was published, Firestone vaulted to prominence as a leading theorist of the second wave of feminism that crested in the 1960s and '70s. But unlike leaders such as Betty Friedan and Gloria Steinem, who made legal equality for women a priority, Firestone preached freedom from "the tyranny of the biological family," envisioning a brave new world in which fetuses were developed in artificial wombs and children were raised in communal households.

"A revolutionary in every bedroom cannot fail to shake up the status quo," Firestone wrote. "And if it is your wife that is revolting, you can't just split to the suburbs. Feminism, when it truly achieves its goals, will crack through the most basic structures of our society."

Subtitled "The Case for Feminist Revolution," Firestone's book was considered essential reading for feminists and in college courses on women's studies.

"No one can understand how feminism has evolved without reading this radical … second-wave landmark," feminist writer Naomi Wolf wrote when the book was reissued in 2003.

Firestone emerged as a radical voice during a fertile era for feminist theory. Her "Dialectic" became a bestseller the same year as Kate Millett's "Sexual Politics," a feminist critique of works by D.H. Lawrence, Henry Miller and Norman Mailer; and Germaine Greer's "The Female Eunuch," which examined history, literature, biology and popular culture.

Some feminists believed that Firestone "had found the solution" to sexual inequality, according to Ruth Rosen in "The World Split Open" (2000), a history of the modern women's movement. But other feminists were incensed by her ideas, particularly because, Rosen wrote, "Firestone seemed to accept men as the normative human being, rather than demanding that society accommodate — and honor — women's important biological contribution as the bearers and rearers of children."

One of six children of Orthodox Jewish parents, she was born Shulamith Feuerstein in Ottawa, Canada, on Jan. 7, 1945. Her family later changed its last name to Firestone. She grew up in Kansas City, Mo., and St. Louis.

Seeing how the civil rights and antiwar movements treated women as second-class citizens, she co-founded three feminist organizations: New York Radical Women, the Redstockings and New York Radical Feminists. She also edited three important collections of feminist writing, beginning in 1968 with "Notes from the First Year."

When "Dialectic" was published in 1970, it won high praise in the New York Times, where critic John Leonard wrote: "A sharp and often brilliant mind is at work here."

As the book rose on bestseller lists, however, its author receded from public view. She rejected the demands of celebrity and was in and out of psychiatric hospitals.

"She wasn't prepared for the crush of publicity and notoriety that came her way," her sister said in an interview Friday. "She pulled away from all that public fame. She was a very interior person."

Firestone is also survived by her mother, Kate Firestone Shiftan; two brothers, Ezra and Nechemia; and a sister, Laya Firestone Seghi.

Her hospitalizations inspired "airless Spaces," a 1998 collection of short stories. The back cover of the book alludes to her personal struggle: "Refusing a career as a professional feminist," it read, "Shulamith Firestone found herself in an 'airless space' — approximately since the publication of her first book 'The Dialectic of Sex.'"

Around this same time, Firestone reneged on an agreement to allow "Dialectic" to be reissued as part of a series of writings by major feminist thinkers.

Feminist writer Jennifer Baumgardner, who planned to oversee the series, was shocked. "I sputtered something about how my generation should have access to the book, that it could change lives and consciousness, and didn't she care about that?" Baumgardner wrote in Dissent magazine in 2002.

"Ifyour generation really wants it," Firestone told Baumgardner, "there are a few old copies available on Amazon.com. I don't feel a responsibility to bring out the book just because you want it. I'm very sorry."